Sat, Dec 29

I assume this is similar to T57939 (in that render pipeline is not covered by DEG)BKE_animsys_evaluate_all_animation() is already done here as an extra evaluation step.
However mask deformations are not actions, so it seems extra evaluation 'by hand' is needed... see D4140 for a potential fix...

Dec 6 2018

If I change the render layer in the compositing tab to sceneB and then back to sceneA, it works as it should in 2.8.
However, if I then save the file and open it again the same problem occurs until I update the node again.

Agree this is a little weak.
It is not a bug though, it is still working as intended.
Not having a Compositing output Node used to complain and refuse to render, but that rule was softened in the fix for T35269.

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Sep 22 2018

It does work when you are using the Defocus Node, using the Z-Buffer of the current scene. Could be that this is the only node that does actually trigger a render update, but then the Auto Render button would be quite misleading indeed.

Thanks for the report, but this is a known limitation. One day we would need to implement more proper color management and such for file output (including manual saving from the editor, render output, nodes...). For until then considering it a TODO.

Sep 6 2018

This is interesting, for sure. I'm not sure that in a typical compositing workflow if it would be extremely useful, but from the examples posted, it certainly looks like you can do some very artsy, creative stuff with it.

It might have something to do with it. This was pretty much the closest workaround solution I came up with, even then there's still some compromise. Is it a compositor/viewport problem or is this just normal for some effects in Blender? Sorry, I'm still fairly new to Blender.